The invention relates to contamination control and measurement and dose control and measurement, for example, in relation to a lithographic projection apparatus and method.
The term xe2x80x9cpatterning devicexe2x80x9d as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning device include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;
A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically directs itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper or step-and-repeat apparatus. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCT patent application WO 98/40791, both incorporated herein by reference.
In a lithographic apparatus the size of features that can be imaged onto a substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation in the range of 5 to 20 nm, especially around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray and possible sources for that radiation include, for instance, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings. Apparatus using discharge plasma sources are described, for example, in W. Partlo, I. Fomenkov, R. Oliver, D. Birx, xe2x80x9cDevelopment of an EUV (13.5 nm) Light Source Employing a Dense Plasma Focus in Lithium Vaporxe2x80x9d, Proc. SPIE 3997, pp. 136-156 (2000); M. W. McGeoch, xe2x80x9cPower Scaling of a Z-pinch Extreme Ultraviolet Sourcexe2x80x9d, Proc. SPIE 3997, pp. 861-866 (2000); W. T. Silfvast, M. Klosner, G. Shimkaveg, H. Bender, G. Kubiak, N. Fomaciari, xe2x80x9cHigh-Power Plasma Discharge Source at 13.5 and 11.4 nm for EUV lithographyxe2x80x9d, Proc. SPIE 3676, pp. 272-275 (1999); and K. Bergmann et al., xe2x80x9cHighly Repetitive, Extreme Ultraviolet Radiation Source Based on a Gas-Discharge Plasmaxe2x80x9d, Applied Optics, Vol. 38, pp. 5413-5417 (1999).
EUV radiation sources may require the use of a rather high partial pressure of a gas or vapor to emit EUV radiation, such as in discharge plasma radiation sources referred to above. In a discharge plasma source, for instance, a discharge is created in between electrodes, and a resulting partially ionized plasma may subsequently be caused to collapse to yield a very hot plasma that emits radiation in the EUV range. The very hot plasma is quite often created using Xe, since a Xe plasma radiates in the EUV range around 13.5 nm. For efficient EUV production, a typical pressure of 0.1 mbar is used near the electrodes of the radiation source. A drawback of having such a rather high Xe pressure is that Xe gas absorbs EUV radiation. For example, 0.1 mbar Xe transmits over 1 m only 0.3% EUV radiation having a wavelength of 13.5 nm. It is therefore required to confine the rather high Xe pressure to a limited region around the source. To achieve this the source can be contained in its own vacuum chamber that is separated by a chamber wall from a subsequent vacuum chamber in which collector and illumination optics may be provided.
Molecular contamination by, for instance, carbon on optical components in a lithographic projection apparatus (e.g. grazing incidence and multi-layer mirrors in a EUV lithographic projection apparatus) is a significant problem. For example, contamination of reflective elements in an EUV lithographic projection apparatus can be caused by the presence of hydrocarbons and secondary electrons that are generated by EUV illumination. A further problem is how to monitor the dose of radiation from a source and the amount of contamination that gathers on an optical component.
Accordingly, it would be advantageous to reduce the amount of contamination caused by illumination of radiation, particularly EUV radiation, on optical components in a lithographic projection apparatus. It would also be advantageous to provide a lithographic projection apparatus in which optical elements, particularly reflective elements used with EUV radiation, are shielded against secondary electrons and in which at the same time the attraction of positively charged particles is prevented. It would also be advantageous to provide a technique for measuring dose and contamination and to reduce the amount of sputtering on optical components by positive ions.
According to an aspect of the invention, there is provided a lithographic projection apparatus comprising:
a radiation system to form a projection beam of radiation, from radiation emitted by a radiation source;
a support structure constructed to hold a patterning device, to be irradiated by the projection beam to pattern said projection beam;
a substrate table constructed to hold a substrate; and
a projection system constructed and arranged to image an irradiated portion of the patterning device onto a target portion of the substrate,
a shield means for generating an electromagnetic field so as to prevent secondary electrons formed during irradiation to become incident on an object to be shielded, said shield means comprising:
an electrode in the vicinity of the object, and
a voltage source connected to the object and/or the electrode for providing a voltage to the object relative to the electrode,
wherein the radiation source is adapted to be operated in a pulsed manner between a high state and a low state, and
the lithographic apparatus comprises synchronization means for providing a time varying voltage to the object and/or to the electrode in synchronism with the radiation source, the time varying voltage imparting a repetitive negative potential to the object relative to the electrode.
Molecular contamination can be reduced by repelling secondary electrons from optical components. Thus, secondary electrons are repelled by a negative (relative) potential pulse from the surface of the object in the radiation beam, or are drawn away by the positive (relative) potential of the electrode. Reduction of molecular contamination and, as discussed below, electron flux measurements can use an electric field to repel electrons from an optical component.
In an embodiment, the lithographic projection apparatus may be provided with a time varying voltage that imparts the repetitive negative potential to the object relative to the electrode for a time period sufficient for transporting substantially all the secondary electrons formed during irradiation away from the object. In this way, secondary electrons are swiftly and substantially completely removed upon generation, as electrons which are present above the irradiated surface of the object can enhance molecular contamination of the surface of the object. For example, when the negative potential pulse is provided sufficiently long after the end of an EUV radiation pulse on an object to allow the (freed) secondary electrons to migrate from the object to the electrode, an electron cloud may not be not present.
In an embodiment, the lithographic projection apparatus may be provided with a negative potential pulse that is applied during a time of between 0.01 microseconds and 10 microseconds, preferably 0.1 microsecond. This time span is long enough to properly repel the secondary electrons which are relatively light. It is however, short enough so as not to cause acceleration and attraction of heavier particles with a positive charge towards the object to be shielded.
In an embodiment, the lithographic projection apparatus may be provided with a negative potential pulse between 0 V and xe2x88x921000 V, preferably xe2x88x92100 V. With such a voltage, secondary electrons can be drawn off from the object in the radiation beam or repelled therefrom such that they only cross the surface of the object once, and hence reduce the amount of contamination caused thereby.
In an embodiment, the voltage source is connected to the object. Direct connection of the negative voltage to the object causes secondary electrons to be rapidly expelled away from the object without a chance of recapture. The positive electrode may be placed at a larger distance from the object or may be formed by the wall of the vacuum chamber.
In an embodiment, the voltage source is connected to the electrode in the vicinity of the object. By applying a positive voltage to the electrode, negatively charged particles such as secondary electrons can be pulled away from the object.
In an embodiment, the negative potential may be applied in phase with the high state of the radiation source. Advantageously, secondary electrons that are generated during a radiation pulse are repelled from the surface on which the radiation impinges by a relatively short negative voltage pulse at the moment when they are formed. Hence the possibility that they return to the surface of the object and cross said surface for a second time is reduced such that the chances of contamination decrease.
In an embodiment, the phase difference between application of the negative potential and the high state of the radiation source may be arbitrary. It is not always necessary to drive the radiation source and provide the pulsed voltage exactly at the same time. An arbitrary phase difference between the radiation pulse and the voltage pulse is also acceptable.
In an embodiment, the repetitive negative potential may be succeeded by an associated positive potential. An electric field will also accelerate positive ions towards an optical component. The resulting ion bombardment can lead to sputtering of the surface of the optical component. Applying a positive potential overcomes the problem that positively charged ions will gain momentum and, particularly in a low pressure environment, move towards the surface of the object to be shielded. The reason is that the relatively heavy ions will experience the time averaged field, which in this case will approximate to zero. The relatively light secondary electrons, on the contrary, will conform to the momentarily present field and thus be removed by the negative voltage pulse.
In an embodiment, a measuring device configured to measure the current generated by secondary electrons in the electrode may be provided. The dose of radiation from a source and the amount of contamination that gathers on an optical component can be monitored by measuring the electron flux from the optical component. An amount of secondary electrons collected is a measure for the dose of radiation and the amount of contamination. This measure may be easily determined using current measuring means connected to the electrode. The current can also be measured at the object.
According to an aspect of the invention, there is provided a method of manufacturing an integrated structure by a lithographic process comprising:
providing a radiation system to form a projection beam of radiation, from radiation emitted by a radiation source, wherein the radiation source is adapted to be operated in a pulsed manner between a high state and a low state;
providing a support structure constructed to hold patterning means, to be irradiated by the projection beam to pattern said projection beam;
providing a substrate table constructed to hold a substrate;
providing a projection system constructed and arranged to image an irradiated portion of the patterning means onto a target portion of the substrate;
generating an electromagnetic field so as to prevent secondary electrons formed during irradiation to become incident on an object to be shielded;
operating the radiation source in a pulsed manner between a high state and a low state; and
providing a time varying voltage to the object and/or to an electrode provided in the vicinity of the object in synchronism with the radiation source, the time varying voltage imparting a repetitive negative potential to the object relative to the electrode.
Although specific reference may be made in this text to the use of the apparatus according to an embodiment to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.